The present invention relates to a method for producing hollow articles, and more especially to a method for producing biaxially oriented containers from thermoplastic resinous materials.
It is recognized that the strength properties of thermoformed plastic containers and recepticles are materially enhanced when the thermoplastic materials utilized to produce these articles are molecularly oriented. Such molecular orientation can be provided by the use of pre-oriented stock materials; however, it is more preferred to impart the molecular orientation to the final articles by including appropriate steps in the article-forming method.
The most beneficial strength properties are achieved when the plastic material in the walls of the final article has been biaxially oriented in a uniform or balanced manner, i.e., the material has been oriented by stretching to the same extent in both coordinate axes of the plane of the material and is characterized by very low birefringence values. However, the accomplishment of this balanced biaxial orientation is a complex task within the context of producing a hollow article of varying cross-section by means of a thermoplastic technique.
Numerous proposals have been made for the production of hollow articles from thermoplastic materials which exhibit a certain degree of molecular orientation. These proposals generally involve a series of separate steps, namely, the formation of a pre-form, a preliminary stretching of the pre-form, and finally a pressure molding step to cause the stretched pre-form to conform to the final desired shape.
The step of forming the pre-form, or parison, as it is often called, is typically carried out in either one of two ways. One method is to form a tubular parison by an extrusion process which is then further processed directly from the extrusion process. In another and commercially significant method, the tubular parison is formed by an injection molding process, after which it is cooled, and then the parison is reheated prior to carrying out the subsequent process steps.
Preliminary stretching of the pre-form or parison is typically carried out by physically engaging the parison with a mechanical stretching device, usually a plug assist, and then moving the stretching device axially with respect to the parison, or vice versa. In this type of process, preliminary stretching is usually done while the plastic material is at its orientation temperature, so that significant orientation takes place in the axial or longitudinal direction. The stretched parison is then subjected to the final blowing step while the plastic material is still at its orientation temperature, and in this way, orientation in the lateral direction or radial direction is produced. The following U.S. Pat. Nos. are illustrative of such methods:
3,311,684 PA1 3,470,282 PA1 3,662,049 PA1 3,754,851 PA1 3,849,530 PA1 3,944,530 PA1 3,949,038 PA1 4,042,657 PA1 4,144,298 PA1 4,151,250 PA1 4,153,667 PA1 4,188,357 PA1 3,767,747 PA1 3,781,395 PA1 Re.29,045 PA1 Re.29,065 PA1 3,882,213 PA1 3,949,034 PA1 3,944,642 PA1 3,978,184 PA1 4,070,428
In another similar process, the parison is subjected to a preliminary blowing step in order to produce a pre-form blown article having a shape generally the same as the final desired article, but smaller in dimensions. This preliminary blowing step is typically carried out while the polymer material is at a temperature above its orientation temperature. Multiaxial orientation is then achieved when the preform blown article is subjected to the final blowing step, i.e., with the polymer at its orientation temperature. Such a process is described in the following U.S. Pat. Nos.:
Other processes apply a separate biaxial orientation step to a film or sheet of thermoplastic material and then subject the oriented sheet to a thermo-forming process, such as vacuum-forming or blow-molding. See, e.g., U.S. Pat. Nos. 4,118,454 and 4,234,536. However, in such processes, the orientation in the polymer sheet is adversely affected during the thermo-forming stage.
Still other specialized processes have been proposed which are intended to produce oriented hollow articles from flat stock material and/or from pre-formed, planar parisons. These processes involve either the formation of a pre-form by means of a mechanical stretching device such as a plug assist, (See, e.g., U.S. Pat. Nos. 3,667,049, 3,737,494, 3,814,784 and 3,966,860), and/or the injection molding or forging of a pre-formed blank (See, e.g., U.S. Pat. Nos. 3,471,896 and 3,739,052). Another process involves feeding two sheets of plastic into a mold at their orientation temperature, sealing the edges and blowing at pressures between about 50 and 150 psig. Some orientation is said to take place during molding, but primary orientation is carried out in separate steps or a pre-oriented material is used.
There exists in the industry a great and long-felt demand for a plastic container which can be used to satisfactorily package carbonated beverages. Such a container must satisfy several strict criteria. It must first of all have sufficient strength to safely withstand the internal pressure coupled with the rough handling conditions to which containers of this type are subjected during bottling and shipping as well as by the ultimate consumer. Furthermore, the container must be capable of maintaining carbonation during periods of prolonged shelf life. In addition, the container must be capable of being mass produced at an efficient rate and from relatively inexpensive materials.
No such product is presently available on the market. Beer and other malt beverages have not yet been packaged in plastic bottles. Soft drinks are beginning to appear in plastic bottles, particularly in the 1 or 2 liter size; however, these bottles, made of polyethylene terephthalate (PET), do not provide shelf life as long as is desired. Most significantly, it is apparent from the foregoing review of the state of the art that none of the heretofore proposed processes is capable of efficiently mass producing such containers at a sufficiently low cost. These processes involve a multitude of separate steps, in each case requiring the transfer of a parison or pre-form from one stage to another, usually in different pieces of apparatus. Furthermore, the apparatus pieces usually have moving parts, such as a plug assist. The process used to produce the commercially available soft drink bottles employs injection molding to produce the parison, and is quite clearly a time- and cost-inefficient process.
Moreover, the prior processes, especially those which injection mold a parison, are obviously limited in the choice of polymeric materials which can be employed to produce the containers. For example, the injection molding of parisons having multiple layer wall configurations is a complex operation. In addition, orientation of multi-layered structures involves many unpredictable factors.